Turns out genfb_softc must be at the start of its own device_private,
so we can't have it in the middle of a structure, and certainly can't
have potentially multiple genfbs per graphics device. Failing to do
this is why entering ddb didn't work with an i915drmkms console.
Instead of putting a genfb_softc into each drm_fb_helper, put a
device_t into each drm_fb_helper and create a new intelfb(4) device
for each Intel framebuffer. This will be more flexible, too, in case
we want accelerated framebuffers later on. (XXX Need to adapt the
radeon code, which I'm disabling until that happens to avoid breaking
the build.)
While here, defer disabling VGA, vga_cndetach, and genfb_attach until
we're actually ready to do them all together along with the mode
switch. This should reduce the amount of time during which the
screen is blanked, in case it hangs.
Screen blanks on boot on the Ivy Bridge system with
DRM error in cpt_serr_int_handler: PCH transcoder A FIFO underrun
But after that everything is OK. Appears to be an upstream problem.
To investigate...
I think there's a cache flushing issue somewhere -- there are little
display artefacts on my T60.
